A polyvinyl acetal is produced by acetalization reaction of a polyvinyl alcohol (hereinafter, sometimes abbreviated as “PVA”) and an aldehyde compound in water under acidic conditions. Since a polyvinyl acetal film is robust and has a unique structure where it has both hydrophilic hydroxyl groups and hydrophobic acetal groups, it has been used for various applications. Thus, a variety of polyvinyl acetals have been proposed. Among others, a polyvinyl formal produced from a PVA and formaldehyde, a polyvinyl acetal in the narrow sense produced from a PVA and acetaldehyde, and a polyvinyl butyral produced from a PVA and butylaldehyde are commercially important.
In particular, a polyvinyl butyral has been widely used as an interlayer film or the like for a laminated glass for an automobile and a building, and it is especially commercially important.
Meanwhile, a polyvinyl acetal has problems such as tendency to coloration by heating; and tendency to formation of foreign materials (undissolved materials) in a polyvinyl acetal film. To solve these problems, various means have been proposed.
Patent Reference Nos. 1 and 2 have described a method for inhibiting coloration of a polyvinyl acetal by acetalization at a particular hydroxide ion concentration under high temperature and pressure. Patent Reference No. 3 has described a method for inhibiting coloration of a polyvinyl acetal by adding a reducing agent after acetalization reaction and subsequent neutralization. However, a film produced using a polyvinyl acetal according to any of the methods described in Patent Reference Nos. 1 to 3 has tendency to contain foreign materials. Patent Reference No. 4 has described a method for inhibiting generation of coarse particles by adjusting a slurry concentration in resin particles obtained, in neutralization after acetalization reaction. Patent Reference No. 5 has described a method for inhibiting generation of coarse particles by regulating relationship between an acid catalyst and a surfactant used in acetalization. However, a film produced using a polyvinyl acetal according to any of the methods described in Patent Reference Nos. 4 and 5 has tendency to contain foreign materials. The film tends to be colored by heating. Thus, a polyvinyl acetal by which all of the above problems can be solved has been strongly desired.
Recently, various high performance products have been developed in applications of interlayer films for a laminated glass. For example, there has been disclosed an interlayer film for a laminated glass wherein a plurality of polyvinyl acetal layers having a different content ratio between a polyvinyl acetal and a plasticizer are laminated, in order for an interlayer film for a laminated glass to be highly sound-insulative (see, for example, Patent Reference Nos. 6 and 7). In the multilayer interlayer film for a laminated glass, a polyvinyl acetal in which each layer contains a different amount of average residual hydroxyl groups has been used, for allowing each layer to contain a different amount of a plasticizer.
In the light of a production cost, an interlayer film for a laminated glass is generally produced using an extruder. The above multilayer interlayer film for a laminated glass is produced by coextrusion. When an interlayer film for a laminated glass is produced by such a process, the process forms not only a certain amount of an end-remnant (trim) in a film end, but also an off-spec product, which cannot be used as a practical product due to its uneven composition and/or thickness.
When a trim or an off-spec product of a monolayer interlayer film for a laminated glass is recycled, there have been problems such as coloration of a film product by heating and generation of foreign materials (undissolved materials) in a film product. When a trim or off-spec product of a multilayer interlayer film for a laminated glass in which each layer has a different composition is recycled, there has been, in addition to the above problems, a problem of transparency reduction due to incompatibility between the components. In particular, when the above sound-insulative multilayer interlayer film for a laminated glass is recycled, it has been difficult to increase compatibility between polyvinyl acetals having different amounts of average residual hydroxyl groups constituting each layer. Thus, an interlayer film for a laminated glass produced has a problem of poor transparency.
In the light of energy saving and effective utilization of resources in recent years, yield improvement in the overall film-producing process has been a key issue. Thus, there has been needed to allow for recycling a multilayer interlayer film for a laminated glass comprising layers with different compositions, which has been difficult. That is, solving the above problems has been desired.